When objects are detected using radar signals, the movement of the objects generates a Doppler frequency shift of the reflected radar signal. Especially in pulse compression radar systems, in which matched filters (also referred to as receiver filters or radar compression filters) are used, such filters are set up in a complementary manner to the transmitted radar pulse. When Doppler shift due to moving objects occurs, the reception characteristic of the radar compression filter is slightly mismatched to the received radar pulse. This leads to a reduced detection performance, accuracy and possibly to erroneous range measurements of the detected object, if the Doppler frequency is not accounted for.
The behavior of a radar signal with regard to Doppler shift can be observed by the use of the ambiguity function. The ambiguity function is the correlation function of the transmitted and received radar echo signal (time delay) after passing through the radar compression filter considering different Doppler shift frequencies. Therefore, the ambiguity function is a three-dimensional function.
So far, the ambiguity function of received radar signals though is not measured, but merely simulated. At present, therefore, it is not possible to accurately determine the behavior of different radar signals, especially of different radar pulse shapes with regard to Doppler shift when considering the entire radar system including filter stages, power amplifier, signal processing etc.
The document U.S. Pat. No. 5,442,359 B1 shows an apparatus and method for mitigating range-Doppler ambiguities in pulse-Doppler radars. Although the system described therein shows options for reducing the effect of observed ambiguities, it does not allow for determining the ambiguity function of a received radar signal itself.
What is needed, therefore, is an approach for measuring the ambiguity function of radar signals.